Magneto inductive flow measuring devices are widely used in process and automation technology for fluids having an electrical conductivity of, for instance, 5 μS/cm. Corresponding flow measuring devices are sold, for example, by Endress+Hauser in the most varied of forms of embodiment for different fields of application under the mark, PROMAG.
This measuring principle rests on Faraday's law of magnetic induction and is described in various publications. By means of a magnet system secured on a measuring tube subsection, a magnetic field of constant strength is produced perpendicular to the flow direction of the conductive fluid. As a result, ions present in the flowing fluid are deflected in opposite directions. The electrical voltage occurring from this separation of charges is sensed by means of at least two measuring electrodes secured at or in the measuring tube subsection. The sensed voltage is proportional to the flow velocity of the fluid and therewith proportional to volume flow.
Correspondingly, a magneto inductive flow measuring device applied in connection with the solution of the invention includes at least the following components: a measuring tube, a magnet system and at least two measuring electrodes. Furthermore, at least one electronics unit is required for signal registration, evaluation and/or power supply, and, in support, likewise a housing, which bounds and protects against the environment the measuring tube subsection with the measuring electrodes as well as at least one additional component of the apparatus, which is secured on the side of the measuring tube facing away from the fluid. This additional component can include besides the magnet system also any cable for signal conduction and located in the direct vicinity of the measuring tube.
If the electronics unit is located in the direct vicinity of the measuring tube subsection—one then speaks of a compact construction—the electronics unit can be accommodated in the same housing as the measuring tube subsection with the measuring electrodes and the magnet system. Otherwise, a separate housing is used. The invention relates to both configurations.
The housing for a flow measuring device should ideally be cost effectively and simply manufacturable. Furthermore, it is advantageous, when sensitive contact locations and cable connections can be fixed in position within the housing, since these can otherwise easily break in the face of repeated vibrations. Known for fulfillment of these requirements is a solution wherein the housing is manufactured by a direct embedding in potting compound. An example of this is described in the European publication, EP1522828A1.
However, a direct embedding of all device components in potting compound also presents many method related, potential problems. On the one hand, most potting methods involve high pressures and/or temperatures. These place high requirements on the respective device component. Due to high temperatures, for example, plastic parts can melt and soldered locations can oxidize. High pressures can, in turn, have the result that individual components are torn from their intended positions. Moreover, the composition of the potting compound must be so selected that sufficient wetting of the respective device components is assured.
In the context of a magneto inductive flow measuring device, of concern is, for example, the contacting of the measuring electrodes as well as cables serving for signal conduction and/or supply and usually integrated into the housing or connecting the measuring electrodes and the magnet system with the electronics unit. Another affected component is the magnet system, which usually is likewise composed of a number of components, including at least two field coils and at least two pole shoes.
A further disadvantage of the direct embedding in potting compound is that potted device components are subsequently only accessible with difficulty, since the potting compound surrounds all device components and occupies all accessible spaces in between them. In the context of a magneto inductive flow measuring device, this relates again especially to the measuring electrodes, the magnet system, as well as any additional components, such as the already mentioned cable, secured in the direct vicinity of the measuring tube subsection.
This can especially be disadvantageous in the case of failure of one of the potted components of the flow measuring device. Often, it then makes more sense to replace the entire flow measuring device than to perform a repair.